1. Field to which the invention relates:
The present invention relates to methods and systems for removing tritium from a gas, as air.
2. The prior art:
In many nuclear- and plasma-physical experiments and work, gaseous tritium (T.sub.2) is produced. Since this is known to be radioactive and, thus, a health hazard, generally it must be removed from the atmosphere in the relevant work-area or the like.
It is known to withdraw tritium-containing air out of work areas, glove-boxes, or the like by means of a blower, and to convert the tritium, with oxygen, into tritium-containing water in a catalytic furnace which may contain CuO, Pd or Pt as a catalyst. The resulting water is then absorbed in a molecular sieve. Conventional modern tritium-separating systems contain, in addition to the catalytic furnace and the molecular-sieve, which are the main components, also heating devices, cooling devices, heat-exchangers and the like.
It is possible in this way to achieve a final concentration of some 10.sup.-5 Ci per cubic metre of air. In practice, however, one must often be content with some 10.sup.-4 Ci/m.sup.3.
The factors governing the minimal obtainable tritium concentration in the purified air are still largely uncertain. The partial water-vapour pressure in the molecular sieve, and the yield from catalytic oxidation, are assumed to be important. Even with a 1% loading of a molecular sieve, the partial water-vapour pressure at 20.degree. C. for optimum performance amounts to about 10.sup.-7 torrs which corresponds in relation to THO to a tritium activity of 2.10.sup.-4 Ci/m.sup.3 of air. This immediately indicates a serious disadvantage of modern purification technology: since the humidity in the ambient air is absorbed from the molecular sieves simultaneously with the tritium-containing water the loading limit of the molecular sieves (about 1%) is soon reached. The molecular-sieve columns must therefore either be made correspondingly large or must be frequently regenerated, and this leads to large quantities of contaminated water. With incomplete catalytic oxidation of the tritium into water, gaseous tritium remains. This passes unimpeded through the molecular sieves and is thus present as an inadmissible output-air activity.
Attempts have been made to eliminate the disadvantages of the above-mentioned methods by cooling the molecular sieves with liquid nitrogen and with novel noble-metal catalysts, but the results have not been satisfactory.